The invention relates generally to metal-ceramic composite material (cermets), combustion synthesis, and infiltration, and more particularly to certain metal-ceramic composites including AlN-Al and methods for making same.
A variety of refractory ceramic materials including nitrides, nitride-oxide and carbide-oxide composites have been produced by combustion synthesis of powder compacts. The process uses heat evolved during spontaneous chemical reactions between mixtures of solids, solids and liquids or solids and gases produced as a combustion wave initiated by an ignition source rapidly propagates through the compact. The key to self-propagating high temperature synthesis (SHS) is that once initiated, highly exothermic reactions will become self-sustaining and will propagate through the reactant mixture in the form of a combustion wave. As the combustion wave (front) advances, the reactants are converted to products. A major advantage of SHS as a process for the synthesis of materials is the energy savings associated with the use of self-sustaining reactions.
Various carbides, borides, selenides, silicides and sulfides can be produced by solid-solid reactions. The starting powders, either metal and metal or metal and nonmetal, are mixed in stoichiometric proportions and cold pressed into a powder compact, typically cylindrical in shape. A heated tungsten coil (or other ignition source) ignites the top surface of the compact. The combustion wave moves rapidly down the compact and leaves behind the refractory product Various nitrides and hydrides can similarly be produced by solid-gas reactions. The metal powder compact is placed in a chamber and nitrogen or hydrogen gas at a suitable pressure, typically about 0.1 MPa (1 atm) or greater, is introduced prior to igniting the compact. The gas pressure must equal or exceed the dissociation pressure of the nitride or hydride at the adiabatic temperature. Some materials require high pressures; e.g. AlN is formed at 14 MPa and Si.sub.3 N.sub.4 at 50 MPa.
Metal powders including Al, Si, Ti, and Zr have been combusted in nitrogen gas to form refractory nitrides. The processes are rapid and require no high temperature furnaces. However, when combustion of most metals is carried out at 1 atmosphere pressure of nitrogen, the yield may be low (under 50%). Insufficient nitrogen fills the pore space of a cold pressed compact of metal powders to achieve full conversion at 1 atm, and molten metal at the wave front restricts flow of nitrogen from outside the compact. In some cases a solid source of nitrogen such as NaN.sub.3 can be used; e.g. ZrN, TiN, HfN, and YN have been synthesized by this method. However, AlN, Si.sub.3 N.sub.4 and BN cannot be formed at 1 atm of pressure using a solid nitrogen source because of high dissociation pressures. Therefore, it is necessary to perform the combustion process in nitrogen gas under high pressure, about 10-100 MPa (100-1000 atm).
The major disadvantage of combustion synthesis is that the product is a very porous (about 50% dense) and tightly bonded solid, or a powder. The porous solid may be useful as formed, or can be easily attrited into a powder. It is generally difficult to form a fully dense product by combustion synthesis. Typically the combustion process is carried out with the simultaneous application of high external mechanical pressure. Pressure techniques include uniaxial rams, explosive compaction, isostatic pressing, and application of shock waves generated by gas guns.
Ceramic-metal composite materials (cermets) generally combine the hardness and light weight of ceramics with the toughness of metals. U.S. Pat. No. 4,605,440 issued Aug. 12, 1986 to Halverson et al. describes boron carbide-reactive metal composites, particularly B.sub.4 C-Al composites, and methods for making same. The process achieves conditions for liquid phase sintering of the metal and ceramic (B.sub.4 C) to occur. Prior to heat treatment a variety of consolidation techniques can be used to produce a fully dense composite with negligible porosity. Low temperature and pressure methods such as consolidation of codispersed ceramic and metal powders are preferred to form a green body which is then heat treated to form a fully dense composite with tailorable microstructure.
Infiltration of a molten metal into a ceramic sponge is known and has been carried out by various different methods. U.S. Pat. No. 3,718,441 to Landingham shows a method of forming metal-filled ceramics of near theoretical density by heating in a vacuum a ceramic compact with a filler metal. U.S. Pat. No. 4,585,618 to Fresnel et al shows an infiltration process in which a bulk reaction mixture of ceramic particulates is reacted to produce a self-sustaining ceramic body while in contact with molten metal which moderates the reaction and infiltrates the resulting ceramic body. U.S. Pat. No. 4,718,941 issued Jan. 12, 1988 to Halverson et al describes an improved infiltration process in which a chemically pretreated porous B.sub.4 C or other boron or boride ceramic matrix or sponge is infiltrated with molten aluminum or other metal to form metal-ceramic composites of high density.
Accordingly, it would be beneficial to combine the advantageous features of combustion synthesis and infiltration in a single process to form a variety of metal-ceramic composite materials having high density which are hard, tough and light in weight. Combustion synthesis would allow formation of the ceramics from elemental components at low energy and material cost. Infiltration would provide high density and metal-ceramic reactions would produce desirable microstructures. It is desirable to control the amount of metal phase to tailor the properties of the composite for particular applications.
In particular, aluminum nitride is a ceramic material which exhibits high thermal conductivity, high electrical resistivity, high mechanical strength, and resistance to oxidation and thermal shock. As such it is commercially important for use as electronic substrates and high temperature applications. A method for forming dense AlN or AlN-Al cermet by combustion synthesis of aluminum powder in a high pressure nitrogen atmosphere is described in U.S. patent application Ser. No. 055,475 filed May 29, 1987, now U.S. Pat. No. 4,877,759, issued Oct. 31, 1989. At about 1000 atm pressure, the product is a completely converted AlN compact densified to about 92% of theoretical density. At about 680 atm the product is a cermet of AlN in an Al matrix. It is desirable to produce a high density AlN-Al cermet with controllable amounts of Al in a ceramic matrix.